1. Field of the Invention
The invention relates generally to sensors and, more particularly, to semiconductor electromechanical sensors which include piezoresistors.
2. Description of the Related Art
Semiconductor sensors with piezoresistors are well known. For example, one type of earlier pressure sensor includes a semiconductor wafer which defines a thinned diaphragm member surrounded by a thicker base portion. A piezoresistor formed on or adjacent to the diaphragm and connected in a Wheatstone-bridge circuit, for example, measures flexion of the diaphragm due to applied force. Piezoresistors typically have a resistance which varies with mechanical stress and temperature. As the diaphragm is deflected due to the application of a force, the resistivity of piezoresistors formed on or adjacent to the diaphragm changes. By measuring such changes in resistivity, a determination can be made as to the magnitude of the force applied to the diaphragm.
Other types of prior semiconductor sensors, such as an accelerometer, for example, may include a beam formed from a semiconductor material having a piezoresistor formed in it. The amount of flexure in the beam bears a known relationship to the magnitude of a force such as an acceleration force applied to the device. Flexure of the beam alters the stress applied to the piezoresistor formed in the beam which changes its resistance. By measuring the change in resistance, the amount of beam flexure can be measured. The beam flexure then can be used to determine the applied force.
Referring to the illustrative drawing of FIG. 1, there is shown a cross-sectional view of a typical earlier semiconductor pressure sensor 14 which comprises an n type wafer 16 which defines a thinned diaphragm 18 surrounded by a thicker base region 20. A surface 22 of the wafer 16 overlays both the base region 20 and the diaphragm 18. A p- doped piezoresistor 28 is formed in the diaphragm 18. A first p+ doped interconnect 26, formed in the base region 20, electrically connects one terminal of the piezoresistor 28 to a circuit contact 32, and a second p+ interconnect 24 formed in the diaphragm 18 also is electrically connected to the piezoresistor 28. The first p+ interconnect 26, for example, can be used to connect the piezoresistor 28 to other piezoresistors (not shown) of a Wheatstone-bridge circuit. An insulative layer 30 overlays the surface of the wafer 16. An opening is formed through the insulative layer so that the circuit contact 32 can be disposed in direct electrical contact with the first p+ interconnect 26.
One problem that can be experienced by pressure sensors such as that shown in FIG. 1 as well as sensors employing a beam having a piezoresistor formed therein, is the build up of electrostatic charges. For example, referring to FIG. 1, negative space charges 34 can build up in the wafer 16 in the vicinity of the piezoresistor 28. The amount of space charge 34 can change over time, and this change can cause "drift" in the resistance value of the piezoresistor. The build up of such space charges can be exacerbated by a corresponding build up of positively charged surface charges 36 on the surface of the insulative layer 30 opposite the negative space charge 34.
The illustrative drawing of FIG. 2 shows one possible solution to the space charge problem. A cross-sectional view of a semiconductor sensor 40 is shown in which a thin metal layer 42 overlays an insulative layer 44 on a portion of a diaphragm 45 that overlays piezoresistor 46. The metal layer 42 can be held at a predetermined constant positive potential (v+), for example, thus stabilizing the surface charge. A stabilized surface charge leads to improved stability in induced negative space charge and thus reduces drift.
Referring to the illustrative drawing of FIG. 3, there is shown still another possible solution to the space charge problem. A cross-sectional view of a semiconductor sensor 60 is shown in which both an n type wafer 62 and a metal layer 64 are connected to a positive (reverse bias) voltage source in order to reverse bias the p-n junction formed between the piezoresistor 63 and the wafer substrate and to maintain the wafer and the metal layer at the same voltage. The wafer 62 and the metal layer 64 are separated by an intervening insulative layer 65. Controlling the potential of the wafer, and the metal layer serves to stabilize the space charge surrounding the piezoresistor and thus reduce drift.
Unfortunately, there are certain disadvantages to the use of such a metal layer on a diaphragm surface. For example, a metal, such as aluminum, can suffer from mechanical hysteresis. This hysteresis, which can be induced both through mechanical flexing and/or thermal cycling, can cause change in the strain fields at the piezoresistor, even with no applied force. A change in the strain fields can result in a change in the offset (output) of a sensor employing a Wheatstone-bridge circuit, for example. In addition, the mechanical properties of the metal can permanently change due to flexion of the diaphragm. As a result, diaphragm flexibility can change through repeated flexing. Another problem is that the thermal coefficient of expansion of metals such as aluminum is not well matched to that of semiconductors like silicon. As a consequence, large offset changes may occur with changes in temperature, and the linearity of these offset changes with temperature may also be degraded. Furthermore, metals, particularly aluminum, can suffer from corrosion, which can degrade reliability. Thus, there has existed a need for an improved semiconductor sensor with piezoresistive elements and improved electrical and mechanical stability. The present invention meets this need.